In recent years, fiber-optic chemical sensors, sometimes called optrodes, have been developed to detect the presence and monitor the concentration of various analytes, including oxygen, carbon dioxide, and pH, in liquids and in gases. Such sensors are based on the recognized phenomenon that the absorbance, and in some cases, the luminescence, phosphorescence, or fluorescence of certain indicator molecules are specifically perturbed in the presence of specific analyte molecules. The perturbation of the luminescence and/or absorbance profile can be detected by monitoring radiation that is absorbed, reflected, or emitted by the indicator molecule in the presence of a specific analyte.
Fiber-optic probes relying upon these characteristics position the analyte-sensitive indicator molecule in a light path at a desired measurement site. Typically, the optical fiber transmits electromagnetic radiation from a light source to the indicator molecule, and the reflectance from or absorption of light by the indicator molecule gives an indication of the gaseous or ionic concentration of the analyte. Alternatively, for monitoring other analytes such as O.sub.2, the optical fiber transmits electromagnetic radiation to the indicator molecule, exciting it into phosphorescence, and the level and/or duration of phosphorescence by the indicator molecule serves as an indication of the concentration of that gas in the surrounding fluid. In the prior art probes, the indicator molecules are typically disposed in a sealed chamber at the distal end of the optical fiber, and the chamber walls are permeable to the analytes of interest.
One problem with the known sensing systems of the type described is that the optical fiber and chamber attached to the end of the probe are prone to physical damage. The optical fibers with attached sensing chambers are delicate because they are disposed as an external appendage at the end of the probe, extending distally beyond a catheter through which the probe is positioned inside a patient's circulatory system. Any mishandling of the catheter can easily result in damage to the delicate sensor chamber.
An additional problem with the known sensing systems described above is that the structure of the chambers and probe configuration often encourage the formation of blood clots, or thrombi. Typically the sensors of the prior art contain discrete optical fibers for each blood gas parameter such as O.sub.2, pH, and CO.sub.2. This multiplicity of fibers adds to the diameter of the complete probe and provides interfiber crevices that encourage thrombi formation. Furthermore, the complexity and difficulty of manufacturing multi-fiber probes is well known, due to the small diameters of the fibers and requirements for their arrangement. Even though a bundled optical fiber probe for sensing a plurality of analytes may have a remarkably small overall cross section, its size can still preclude its use in neonatal or pediatric applications in which the patient's veins or arteries are too small in diameter for insertion of the sensor assembly. Thus, prior art multi-analyte sensors fail to effectively deal with several problems.